Mass spectrometers are instruments used to analyze the mass and abundance of various chemical components in a sample. Mass spectrometers work by ionizing the molecules of a chemical sample, separating the resulting ions according to their mass-charge ratios (m/z), and then measuring the number of ions at each m/z value. The resulting spectrum reveals the relative amounts of the various chemical components in the sample.
Electron ionization (EI) is one common method for generating sample ions. In EI, electrons are produced through a process called thermionic emission. Thermionic emission occurs when the kinetic energy of a charge carrier, in this case electrons, overcomes the work function of the conductor. In the vacuum chamber of the gas analyzer, where there may be virtually no gas to conduct heat away or react with the filament, a current through the filament quickly heats it until it emits electrons. The filament may be set to a voltage potential relative to an electron lens or other conductor, and the resulting electric field accelerates the electron beam towards the sample to be ionized. As the electron beam travels through the gaseous sample, the electrons may interact with and ionize and potentially fragment molecules in the sample. The charged particles can then be transported and analyzed using additional electric fields. EI can be performed either in the mass analyzer itself, or in an adjacent ionization chamber. The advantages of each system will be discussed with reference to the prior art below.
One type of mass analyzer used for mass spectrometry is called a quadrupole ion trap. Quadrupole ion traps take several forms, including three-dimensional ion traps, linear ion traps, and cylindrical ion traps. The operation in all cases, however, remains essentially the same. Direct current (DC) and time-varying radio frequency (RF) electric signals are applied to the electrodes to create electric fields within the ion trap. These fields trap ions within the central volume of the ion trap. Then, by manipulating the amplitude and/or frequency of the electric fields, ions are selectively ejected from the ion trap in accordance with their m/z. A detector records the number of ejected ions at each m/z as they arrive.
Ion traps are optimized for a combination of speed, sensitivity, resolution, and dynamic range depending on the particular application. For a given instrument, an improvement in one category is usually made at the expense of another. For example, resolution can generally be increased by using a slower scan, and in the reverse a scan can be performed faster at the expense of resolution. Similarly, sensitivity—especially to less abundant components of a sample—can be increased by trapping and scanning a larger total number of ions in a single scan. However, as the quantity of ions in the trap increases, the coulombic forces between the like-charged ions in the trap cause expansion of the ion cloud. When this occurs, ions at different locations within the cloud perceive slightly different electric fields. Mass spectrometers achieve resolution by ejecting all ions of the same m/z at close to the exact same moment, but when different ions of the same m/z perceive different electric fields, they may eject from the trap at different times. The result may cause broadening of spectral peaks referred to as the “space charge” effect. Space charge may also be caused by collisions when ions strike one another, particularly when large ions strike smaller ions. This increases the kinetic energy of some ions, thus ejecting them out of the ion trap before they would otherwise be removed by changes in the ion trap electrode potential.
Furthermore, specific components of a mass spectrometer may limit various performance specifications of the instrument. For example, a typical channel electron multiplier (OEM), a common type of ion detector, has a dynamic range of 2-3 orders of magnitude, which sets a ceiling for the overall system dynamic range independently of the performance of the mass analyzer. Thus, the design of other components of the instrument need to take these effects into account.
Conventional mass spectrometers have sought to achieve a balance between sensitivity and resolution by optimizing the quantity of ions trapped. For example, mass spectrometers have tried to achieve these benefits by: adjusting the trap loading time, adjusting the ionization time, or adjusting the ionization rate. However, such arrangements still have drawbacks. As a result, there still exists a need for a mass spectrometer that allows for improved control of the rate of ionization, as well as a beneficial balance between sensitivity and resolution, while also minimizing the size of the mass analyzer, the length of mass scans, and the power consumption of the instrument.